In electric arc welding, one of the most popular welding process is pulse welding which primarily uses a solid wire electrode with an outer shielding gas. MIG welding utilizes spaced pulses which first melt the end of an advancing wire and then propel the molten metal from the end of the wire through the arc to the workpiece. Under ideal conditions, a globular mass of molten metal is melted and transferred during each pulse of the pulse welding process. An interruption in the normal operation of a pulse welding process occurs when the molten metal contacts the workpiece before being released from the advancing wire. Consequently, the high voltage pulse welding of over 25 volts is normally used so that the gap between the end of the electrode and the wire is relatively large. This limits the incidence of short circuits and the resulting spatter and puddle disturbance. It is advantageous to have a small gap or arc length less than about 0.20-0.30 inches. However, pulse welding usually requires a substantially higher voltage to assure proper transfer of the molten metal and to reduce short circuits. Nevertheless, the pulse welding process invariably involves a short circuit condition which must be eliminated rapidly to obtain the consistency associated with proper pulse welding. To remove short circuits, it is well known to increase the arc current immediately upon detection of the short circuit. The high arc current causes an electrical necking action to immediately separate the molten metal from the advancing electrode to again establish the arc. A discussion of this well known concept is contained in Ihde U.S. Pat. No. 6,617,549 incorporated by reference herein. Even with this well known short circuit clearance procedure, high voltage is still required for solid wire and the travel rate of the wire must be fairly low. When attempting to use cored wire for pulse welding, the arc voltage must be maintained fairly high, well above 25 volts, to avoid short circuit conditions that are not desired in a pulse welding process. In summary, short circuits cause reduced quality of the weld and reduce the traveling rate of the welding operation, as well as requiring high voltage with its disadvantages. These short circuits are more troublesome when attempting to use the metallurgical advantage of metal cored electrodes.
Short circuits in a pulse welding process affects arc stability, especially at lower voltages where the average arc length is less than about 0.20-0.30 inches. They also cause spatter during breaking of the short circuit. Consequently, pulse welding requires a procedure for clearing of inadvertent, random short circuits. This was done by merely increasing the arc current until the short circuit was cleared. Thus, the pulse welding process required high voltages, greater than 25 volts, to minimize inadvertent short circuits. This resulted in the need to operate at lower travel speeds. Furthermore, spatter and non-uniform weld beads resulted when high voltages and normal short circuit clearing was employed.
Pulse MIG welding primarily uses a solid wire electrode, metal cored wire, or flux cored wire typically shielded with an outer shielding gas. The power source creates a special pulsed output that alternates between a high output, sometimes called the “peak” and a lower output, called the “background.” The peak output is greater than the welding electrode's spray transition current for a duration long enough to form and transfer one droplet of metal from the advancing electrode to the workpiece. Between pulses, the lower background output allows the electrode to advance toward the workpiece and be repositioned in order for the next peak to deposit the next droplet. Under ideal conditions, the pulsed output is maintained such that one droplet transfers from the electrode to the workpiece for each peak without allowing the droplet to bridge the gap causing a short circuit. This condition can be achieved when a sufficiently long arc length is maintained producing a relatively high average arc voltage. For example, pulse welding with a steel electrode running under 90% argon, 10% CO2 is performed with an average voltage greater than about 26 volts.
In practice, there are many advantages when operating a welding process, such as pulse welding at shorter arc lengths. These advantages include lower heat input and better control of the puddle at higher travel speeds. At reduced arc lengths, partially transferred droplets are more apt to bridge the gap between the electrode and the work causing short circuits. As the arc length is reduced, shorting events become more frequent and become harder to clear. Modem pulse welding power sources, such as the POWERWAVE by Lincoln Electric contain a technique to clear short circuits. When a short circuit is detected, the machine's output is increased in a controlled fashion until the short circuit is “pinched” off and the short is cleared. A discussion of this well-known concept is contained in Kawai U.S. Pat. No. 4,889,969, and in Ihde U.S. Pat. No. 6,617,549 incorporated by reference herein. Using this well-known technique, the welding process will remain stable even while occasional short circuits occur. This method allows users to reduce the arc length yet maintain stable operation at lower heat input levels. This improves the fast follow characteristics at higher travel speeds. For the previously cited example, the stable operation point is reduced to a voltage greater than about 23 volts. As the arc length is reduced below this point, shorting events occur quite frequently and may require a significant increase in pinch current in order to break shorts. When the short does break at high current, spatter typically occurs and an accompanying instability will follow as the high current pushes down on the puddle causing an oscillation. This problem is sometimes caused by repetitive shorting. As a short is cleared, another short immediately forms and is difficult to clear.
Cored wires are wires that are comprised of a metal sheath containing a core of metal power and/or slag producing compounds (FCAW-G) and/or compounds that produce shielding gases (FCAW-S). These wires are very advantageous to produce the desired metallurgy of the weld metal and to protect from contamination. Many of these cored wires can be used in a pulse welding process in a fashion similar to solid wires. However, in use of solid wires, these cored wires exhibit an increase in the frequency and severity of short circuits as the arc length is reduced. Indeed, the minimum arc length required for cored wires is higher than the minimum arc length or voltage for a solid wire since pulsing cored wires tends to melt the sheath leaving the core exposed allowing it to dip into the puddle. Thus, the advantage associated with cored electrodes can not be fully employed. There is a need for a pulse welder that can use cored electrodes with a reduced voltage without the problem of repeated short circuiting or where such shorts are cleared efficiently to eliminate their adverse impact.